1. Field of the Invention
The present invention generally relates to an image forming apparatus such as a printer and a copier. More particularly, the present invention relates to an image forming apparatus including a photosensitive body, a transferring device, and a fixing device, the transferring device being configured to transfer a toner image developed on the photosensitive body onto an image recording member, and the fixing device being configured to fix the toner image transferred onto the image recording member by heating and pressing.
2. Description of the Related Art
Conventionally, in a fixing device configured to heat and fix a toner image on the image recording member, a thermal roller method is generally employed. FIG. 1 schematically shows a general configuration of a fixing device 9 and a feeding mechanism nearby the fixing device 9 for feeding an image recording member in a conventional electrophotographic type image forming apparatus.
As shown in FIG. 1, the fixing device 9 includes a heating roller 9a and a pressing roller 9b. The heating roller 9a includes heating means (not shown) such as a halogen lamp within the heating roller 9a. The heating roller 9a generally includes a core bar having a cylindrical shape and a fluorine resin layer or a silicon rubber layer coated on the core bar for improving the releasing characteristics of toner. As the fluorine resin, PFA tubing may be used or PFA may be applied and baked. The pressing roller 9b generally includes a core bar, a heat-resistant silicon rubber layer formed on the core bar as an elastic body layer, and a PFA tubing layer having excellent releasing characteristics wound on the surface of the elastic body layer.
The heating roller 9a and the pressing roller 9b constituting a fixing roller pair are pressed to each other by pressing means (not shown) using a spring, so that the contacting surface between the heating roller 9a and the pressing roller 9b forms a surface due to deformation of the elastic body layer of the pressing roller 9b. Herein, this contacting surface is called a nip section. While the nip section (contacting surface) is being formed, the heating roller 9a and the pressing roller 9b are driven to rotate in a predetermined direction by fixing roller pair driving means and roller driving control means.
By conducting a known electrophotographic process, a toner image visualized on a photosensitive drum 6 is transferred onto a sheet (herein may be referred to as an image recording member) 22 by a transferring device 8. Then, the sheet 22 is fed through a sheet feeding path 12 to the nip section between the heating roller 9a and the pressing roller 9b. The fixing device 9 further includes a fixing device introduction guide member 16 configured to guide the sheet 22 to the nip section. The fixing device introduction guide member 16 is formed so that a downstream end of the fixing device introduction guide member 16 is formed in a straight line on the same plane. By following a series of processes as described above, the toner image formed on the sheet 22 is heated and pressed by the heating roller 9a and the pressing roller 9b so that the toner image is fixed onto the sheet 22.
Unfortunately, in such a fixing device using the thermal roller method, when a thin image recording member (sheet) having high moisture content is used to be printed, the moisture in the image recording member may evaporate because the image recording member is rapidly heated. As a result, the image recording member thermally shrinks, thereby causing a deformation of the image recording member, which may result in developing wrinkles. To overcome the problem, there has been generally adopted a method of preventing wrinkles in which the shape of the heating roller is modified to have a so-called reverse-crown shape so that the center portion of the heating roller becomes thinner than the end portions of the heating roller. By configuring this way, the distortion of the pressing roller at end portions of the image recording member in the width direction (orthogonal to the feeding direction of the image recording member) becomes greater than that at the center portion of the image recording member. Therefore, in the nip section, the image recording member is pushed toward the end portions in the width direction to prevent wrinkles.
Further, Patent Document 1 proposes a technique in which the guide member for guiding the image recording member to the nip section is configured so that the center portion of the guide member is higher than both end portions in the width direction, and accordingly, the center portion of the guide member is closer to the heating roller than the both end portions. By configuring the guide member in this manner, it becomes possible to prevent the both side ends of the image recording member from approaching the center portion of the image recording member. Further, because of the configuration of the guide member, the image recording member is in contact first with the heating roller before being fed into the nip section. As a result, the image recording member is bent in a short curve by the guide member and the heating roller, and a downstream end section in the feeding direction of the image recording member becomes rigid, which helps to maintain a flatness of the image recording member and prevent wrinkles.
Further, Patent Document 2 proposes a technique in which, the shape of the downstream end of the guide member in the height direction is configured to correspond to the shape of an entrance portion of the nip section between the heating roller and the pressing roller. In other words, the center portion of the downstream end of the guide member is higher than end portions of the guide member in the width direction so that the shape of the downstream end of the guide member in the height direction is substantially equal to the reverse-crown shape which is the shape of the entrance portion of the nip section. By configuring this way, the image recording member may be uniformly fed into the nip section to prevent wrinkles.
Further, Patent Document 3 discloses a technique in which the guide member is configured so that the downstream end portion of the guide member includes a lower plane part in the center section and higher plane parts on both sides of the lower plane part in the width direction. The fixing device is provided as a film heating and fixing device including a heating roller and a pressing roller. The heating roller includes a heater and a heat-resistant endless film. The pressing roller includes an elastic body layer made of silicon rubber or the like. In the above documents, by making the shape of the image recording member correspond to the concave shape in the height direction, the end sections of the image recording member in the width direction enter first into the nip section. Therefore, the image recording member is pushed toward the end portions in the width direction, which may effectively prevent wrinkles caused due to a folded portion in the center portion of the image recording member. In the techniques proposed in this document, as described above, the end portions of the image recording member in the width direction are higher than the center portion of the image recording member so that the ends portion of the image recording member enter into the nip section first. To achieve this, from a geometric point of view, the downstream end of the guide member should be lower than a plane including the nip section (nip plane). This configuration of Patent Document 3 is upside-down compared to the configuration of Patent Document 1 where the center portion of the guide member for guiding the image recording member into the nip section is higher than the ends portion of the guide member in the width direction. Therefore, the technique disclosed in Patent Document 3 as a wrinkle prevention technique may be regarded as an extension of the technique disclosed in Patent Document 1.
According to the descriptions disclosed in Patent Documents 1 through 3, wrinkles of the image recording member may be prevented by optimizing the shape of the downstream end of the image recording member when the image recording member is to be fed into the nip portion. However, in electrophotographic type image forming apparatuses, there are many cases where so-called cut sheets are used as the image recording member; in most of these cases, the image recording members are obliquely fed (skew-fed) into the fixing device. Therefore, even if the image forming apparatus employs any of the techniques described above, the downstream end of the image recording member is not always fed into the nip section as it is assumed in the descriptions. On the other hand, when the image recording members are “skew-fed” into the nip section, there is a quite high probability of wrinkles occurring. Therefore, any technique described above alone may not be enough to reduce the wrinkles. To reduce wrinkles of the image recording member, the fixing roller is required to have the reverse-crown shape. However, when the angle of the reverse-crown shape is too large, the image recording member may be excessively pushed toward the end portions in the width direction in the nip section. In this case, the downstream side of the image recording member may be deformed, which may cause waves of the image recording member in the direction orthogonal to the plane of the image recording member on the surface of the image recording member. When the wavy image recording member is fed into the nip section, due to misaligned contact with the fixing roller, the toner image transferred onto the image recording member may be disturbed, thereby causing image disturbance.
It is known from prior art that even when the downstream end of the guide member is in a straight line having a constant height and the image recording member is somewhat skew-fed, by setting a diameter size of the reverse-crown of the heating roller to be about 200 μm, setting the width of the nip section equal to or more than 10 mm without becoming narrower in the sheet feeding direction, and setting the downstream end of the guide member higher than the nip plane so that the guide member becomes closer to the heating roller with respect to the nip plane, wrinkles may be reduced in most cases. Further, by configuring this way, even when a wide-size sheet such as an A3 size sheet and a longer-sized sheet such as a ledger sheet are used, the disturbance of an image due to the reverse-crown of the heating roller may be reduced.
On the other hand, recently, more and more so-called thick coat papers and preprint papers have been widely used as the image recording member in electrophotographic type image forming apparatuses in such as direct mail (DM) marketing due to increasing Print On Demand. Further, there is also a growing demand for using image recording members (sheets) having a larger size and a lower weight due to increasing Book On Demand in publishing markets. For example, in Japan, when an A5 size book is produced, an extended size of sheet larger than a thin A3 size sheet and having a basis weight of 60 g/m2 may be used so that four pages of document content are printed on each of both sides of the sheet, i.e., in total, eight pages of document content are printed on the single sheet. In some areas outside Japan, when manuals of products are produced, four pages of document content are printed on each of both sides of a sheet having a basis weight of 75 g/m2 and size 14 inches (355.6 mm)×18 inches (457.2 mm). In such a case where a thin and large sheet is to be printed, the rigidness of the sheet may become lower and also the influence of the distortion of the nip portion may become larger. Therefore, the image distribution due to the reverse-crown is more likely to be remarkable in appearance. This phenomenon may be reduced by reducing the amount of reverse-crown, but if the amount of reverse-crown is reduced, the margin against the wrinkles may be relatively reduced.    Patent Document 1: Japanese Utility Model Application Publication No.: S56-130954    Patent Document 2: Japanese Patent Application Publication No.: H6-95542    Patent Document 3: Japanese Patent Application Publication No.: 2006-91448